In article ,
C. J. Clegg wrote:

So, my questions are...

Take my answers as best-guesstimates, please, rather than as gospel!

1. How do I determine the overall antenna length that will give me the
most efficient (which is to say, the least inefficient) performance across
the 4 to 9 MHz range?

A range greater than 2:1 means that you're almost certainly going to
run into at least one operating frequency at which the antenna itself
is an extremely difficult load (very low, very high, and/or very
reactive) and that most of your power is going to end up in the
terminating resistor.

One way to evaluate different lengths would be to model out such an
antenna using NEC2/NEC4 or the like. Vary the antenna-length-to-
operating-frequency-wavelength ratio, and evaluate the amount of power
radiated vs. the amount of power dissipated in the termination
resistance at each. Then, given the specific frequencies at which you
actually want to spend most of your time operating, figure out which
length gives you the best overall efficiency for your own operating
conditions.

At a guess - and it's just a guess - I think you might get reasonably
satisfactory results by choosing a length which would give you a
folded dipole that's resonant right in the middle of your 4-to-9
range, or perhaps a bit longer than that. You'd avoid the "too short
to load up efficiently" problem at 4 MHz, as well as the "a full
wavelength long, and thus presenting a high feedpoint Z" problem at 9
MHz.

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?

Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

3. Where can I buy a few noninductive terminating resistors that meet the
power rating determined in (2)?

You could probably stick a bunch of Caddock MP9100 power film
resistors, wired in series, on a chunk of aluminum heatsink and make
it work OK. Caddock sells direct, I believe, and if I recall
correctly Mouser carries many of their parts.

You might ask B&W if they'd be willing to sell a "replacement" for
their termination resistor assembly.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?

Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.

On Tue, 21 Nov 2006 23:15:44 -0500, "C. J. Clegg"
wrote:

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.

Hi OM,

By turns, you've painted yourself into a corner when we add up this
wish list. Worse yet is the complaint you anticipate with:
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.

3dB is hardly they abyss of performance, and, in fact, you would
probably be hard pressed to notice it. If you could, your dream
antenna (for the price you are willing to pay) would automatically
qualify for the dung-heap.

When you lead with your chin with:
It cannot use any sort of antenna tuner
this simply breaks the camel's back. Also, come to terms with there
is also no such thing as a miracle BalUn.

In traditional engineering, there is the adage that a well defined
problem contains its own answer. You can reconcile facing abject
failure by falling back and building a cost/benefit analysis of all
the characteristics of your desired antenna. It should take more than
half an hour to come up with the price tag. You may also discover
that you can live without some of those restrictions and you might be
able to live with a tuner - generations have survived and flourished
under similar circumstances.

73's
Richard Clark, KB7QHC

On Tue, 21 Nov 2006 22:10:19 -0800, Richard Clark wrote:

By turns, you've painted yourself into a corner when we add up this
wish list. Worse yet is the complaint you anticipate with:
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.

When you lead with your chin with:
It cannot use any sort of antenna tuner
this simply breaks the camel's back. Also, come to terms with there
is also no such thing as a miracle BalUn.

Good morning, Richard.

I was hoping to mitigate all of that by restricting the frequency range.
I'm not asking for 1.8-30 or even 4-30, but 4-9.

Can a T2FD antenna not be made 50 percent efficient across that restricted
frequency range?

The 50 percent efficiency floor is somewhat arbitrary but is driven by the
fact that some of the users of this antenna are going to be using power
levels as low as 5 watts. That's hard enough to do with a cut NVIS
dipole, without throwing an inefficient antenna into the mix.

The inability to use a tuner is driven by the fact that eventually, these
antennas are going to need to be usable with ALE radios. I don't know
very much about ALE (yet) but I have a hard time imagining an autotuner
that is consistently fast enough to use with ALE. Also, the users of these
antennas are, like me, going to be paying for them out of their pockets.
Few if any of the potential users that I know of today can afford to buy
an autotuner. I know I can't.

If it can't be done, then it can't be done. I'm realistic enough to
accept that and move on to something else, or drop the idea. It just
seems to me that within the limited frequency range, something like that
should be possible with the right choice of design parameters.

On Wed, 22 Nov 2006 07:10:55 -0500, "C. J. Clegg"
wrote:

I was hoping to mitigate all of that by restricting the frequency range.
I'm not asking for 1.8-30 or even 4-30, but 4-9.

Hi OM,

That has been evident from the beginning. Still and all:
The 50 percent efficiency floor is somewhat arbitrary
Exactly. It is just such arbitrariness along with competing,
conflicting restrictions that leads to self-defined failure tacitly
accepted with:
If it can't be done, then it can't be done.
However, it has been done, and without the unnecessary complications
of forcing ANY design to span an octave without a tuner. Hoping that
a resistor will solve this is dope-slapped with the expectation of
efficiency.

some of the users of this antenna are going to be using power
levels as low as 5 watts
So what is all this angst about efficiency? Pour 100W into any hank
of wire and you will achieve at least that. Or do you mean that the
design you are looking for will be used as a model for others? If
that is the case, more the pity that the requirement of having a $25
tuner is an unreasonable expectation of Amateur radio operators. I
cannot imagine they will spend less on those resistors....

73's
Richard Clark, KB7QHC

On Wed, 22 Nov 2006 12:37:20 -0800, Richard Clark wrote:

So what is all this angst about efficiency? Pour 100W into any hank
of wire and you will achieve at least that. Or do you mean that the
design you are looking for will be used as a model for others?

Good afternoon, Richard.

Actually, what I mean is that some, including me, will be using 5-watt
radios (Yaesu FT-817). If the option of pouring 100W into any hank of
wire was available to all, the issue wouldn't be critical. It's not,
so it is. :-)

more the pity that the requirement of having a $25
tuner is an unreasonable expectation

Remember the ALE requirement. I am only just now becoming convinced that
some autotuners might work for ALE. I'm quite certain manual tuners won't.

In article ,
C. J. Clegg wrote:

Actually, what I mean is that some, including me, will be using 5-watt
radios (Yaesu FT-817). If the option of pouring 100W into any hank of
wire was available to all, the issue wouldn't be critical. It's not,
so it is. :-)

There's another approach to the antenna that you might want to
consider, _if_ the ALE ranges are limited to a few (say, no more than
four or five) and aren't too wide (say, no more than perhaps 5% of the
center frequency).

If your usage pattern fits this model, then you could construct a
multi-wire "fan" dipole array... simply a set of individual wire
half-wavelength dipoles, each cut for the center frequency of a given
ALE range, spread apart physically, and fed from a single coax at a
single feedpoint. They can be spread vertically (e.g. hang the
longest one as a flat-top from a convenient set of trees, suspend the
next-shortest beneath it on 6" spreaders, hang the next-shortest on
another set of spreaders, etc.) or horizontally (run one north/south,
another east/west, etc.).

There will be some amount of coupling/loading between them, so you'll
need to trim them for lowest SWR once they're installed, and you may
find the SWR bandwidth less than you'd get from individual dipoles.
Coupling/loading is greater for the vertically-stacked case, and less
for a star-like horizontal pattern (which takes more space and more
trees or etc., of course).

The nice thing about this approach, if it's suitable for your needs,
is that the losses are quite low. The wire which is resonant on the
band in question loads up and radiates, and the other wires (which
will typically have high, reactive impedances) accept very little
current. No lossy termination resistor is needed.

I've been using a three-wire fan dipole (5" vertical spacing) cut for
40/20/10 for several years, with very satisfactory results.

This approach won't give you continuous coverage of your 4:9 range...
unfortunately I can't suggest any which would, which don't require an
agile tuner/transmatch and which don't introduce high losses in some
portions of the band.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Wed, 22 Nov 2006 15:56:45 -0500, "C. J. Clegg"
wrote:

more the pity that the requirement of having a $25
tuner is an unreasonable expectation

Remember the ALE requirement. I am only just now becoming convinced that
some autotuners might work for ALE.

I seriously doubt that. What you are describing is a sacrifice in
Link Quality for the sake of not using something like a fan dipole
which would immediately satisfy every requirement and fulfill mission.
How this is justified with
multiple-legged dipole with legs cut
for different frequencies (well, I probably could, but it would be
impractical).
forces a new meaning to the word impractical. Perhaps you would care
to elaborate how the simplicity of two extra wires has been trumped.

73's
Richard Clark, KB7QHC

C. J. Clegg wrote:
On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?
Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.



Narrowing the frequency range isn't going to help much with the design.
I'm no expert, but if I read my signs correctly, the T2FD has the same
ups and downs in SWR as does most other multiband antennas. IOW, it
relies on frequency harmonic relationships.

Cebik has a good web page at:

http://www.cebik.com/wire/t2fd.html

Michael Coslo wrote:
C. J. Clegg wrote:
On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?
Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in
the
resistor, at any frequency within my range of 4 to 9, isn't going to
work.
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.



Narrowing the frequency range isn't going to help much with the
design. I'm no expert, but if I read my signs correctly, the T2FD has
the same ups and downs in SWR as does most other multiband antennas.
IOW, it relies on frequency harmonic relationships.

Pah! I should have said wavelengths, not harmonics!!

- 73 de Mike KB3EIA -